Optical hydrogen detector

ABSTRACT

A hydrogen gas detector for detection of hydrogen gas in a gaseous environment. The detector comprises a light/heat source, an optical detector, and an optical barrier between the source and detector. The optical barrier responds to the presence of hydrogen by responsively changing from a first optical state to a different second optical state, whereby transmission of light from the light/heat source through the optical barrier is altered by the presence of hydrogen and the altered transmission is sensed by the optical detector to provide an indication of the presence of hydrogen gas in the gaseous environment.

[0001] This invention was made with Government support under ContractNo. NAS10-98027 awarded by the National Aeronautics and SpaceAdministration. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to the detection ofhydrogen gas in a gaseous environment, and more specifically to anoptical hydrogen gas detector apparatus and hydrogen gas detectionmethod.

[0004] 2. Background of the Invention

[0005] Hydrogen holds vast potential as a commercial source of energy.As reported by the Hydrogen Technology Advisory Panel, (HTAP),“[h]ydrogen will join electricity in the 21^(st) Century as a primaryenergy carrier in the nation's sustainable energy future” (VisionStatement, The Green Hydrogen Report; The 1995 Progress Report of theSecretary of Energy's Hydrogen Technology Advisory Panel,DOE/GO-10095-179 May 1995). The abundance and versatility of hydrogensuggests that it can provide solutions to problems encountered withcurrent fossil fuel energy systems, such as declining domestic supplies,air pollution, global warming, and national security.

[0006] Significant research and development efforts are currentlyunderway to make the widespread use of hydrogen technically andeconomically feasible. These efforts are directed toward creating thebasic infrastructure of a hydrogen economy: production, storage,transport and utilization. An underlying need of each of theseinfrastructural components is the ability to detect and quantify theamount of hydrogen gas present in a gaseous environment. This iscritical not only for health and for human safety reasons, but will berequired as a means of monitoring hydrogen-based technology and for thedevelopment of high-efficiency hydrogen processes. Hydrogen gas sensorsthat can quickly and reliably detect hydrogen over a wide range ofoxygen and moisture concentrations are not currently available, and mustbe developed in order to facilitate the transition to a hydrogen-basedenergy economy.

[0007] Hydrogen is the lightest and most abundant element in theuniverse. As a gas, hydrogen is odorless, colorless, and bums with avirtually invisible flame (an effective odorant and luminant withminimal system and emission impact has not yet been developed). It has alower explosive limit (LEL) of 4% in air, and an upper explosive limit(UEL) of 75%. The minimum self-ignition temperature of a stoichiometricmixture of hydrogen and oxygen is 585° C.

[0008] Although the safety record of the commercial hydrogen industryhas been excellent, it is estimated that undetected leaks were involvedin 40% of industrial hydrogen incidents that did occur (“The Sourcebookfor Hydrogen Applications,” by the Hydrogen Research Institute and theNational Renewable Energy Laboratory, 1998). Emerging hydrogen-basedenergy systems will require hydrogen sensors that are as ubiquitous ascomputer chips have become in current home, office, factory andvehicular environments. This circumstance in turn requires that massivenumbers of hydrogen sensors be readily manufacturable at low cost.

[0009] In this respect, any commercially viable hydrogen detector mustsatisfy the following requirements:

[0010] the detector must be selective to hydrogen in a wide variety ofgaseous environments, including oxygen-rich, high-humidity environmentsfound in fuel cells;

[0011] the detector must have a good signal-to-noise ratio and a largedynamic range;

[0012] the detector should minimize both failures to detect and falsepositives;

[0013] the detector must operate rapidly, inasmuch as high speeddetection is a critical requirement to ensure rapid response topotentially hazardous leaks of hydrogen;

[0014] the detector must have a long lifetime between calibrations, inorder to minimize maintenance requirements and achieve low lifetimecosts with high reliability;

[0015] the detector must be characterized by low power consumption,which is particularly critical for portable instrumentation andpersonnel monitoring device applications;

[0016] the hydrogen detector should be characterized by a high level ofoperational safety, and should not depend on any heated wire, openflame, or spark for its operation; and

[0017] the hydrogen detector must be reliably and reproduciblymanufacturable at high volumes, and be readily available in greatnumbers, at low cost, to achieve ubiquitous monitoring of numerous,dynamically changing and diverse environments.

SUMMARY OF THE INVENTION

[0018] The present invention relates in one aspect to a hydrogen gasdetector for detection of hydrogen gas in a gaseous environment, suchdetector comprising a light/heat source, an optical detector, and anoptical barrier between the source and detector, wherein the opticalbarrier responds to the presence of hydrogen by responsively changingfrom a first optical state to a different second optical state, wherebytransmission of light from the light/heat source through the opticalbarrier is altered by the presence of hydrogen and the alteredtransmission is sensed by the optical detector to provide an indicationof the presence of hydrogen gas in the gaseous environment.

[0019] In another aspect, the invention relates to a hydrogen gasdetector, comprising

[0020] a light source;

[0021] a thermal energy source;

[0022] an optical filter having an optical transmissivity responsive tothe presence and concentration of hydrogen gas in an ambient environmentto which the optical filter is exposed, such optical filter beingdisposed in proximity to the light source so that the optical filter isilluminated with light from the light source, and being operativelycoupled to the thermal source so that the optical filter is heated bythe thermal source;

[0023] a light detector generating an output signal, the state of suchoutput signal being proportional to the intensity of light impinging onthe light detector, and the light detector being disposed inlight-sensing relationship to the optical filter, whereby light from thelight source passing through the optical filter impinges on the lightdetector and generates the output signal as an indication of thepresence and/or concentration of hydrogen gas in the ambientenvironment.

[0024] A further aspect of the invention relates to a hydrogen detectionsystem for monitoring an extended or remote area region for theincursion or generation of hydrogen therein. The hydrogen detectionsystem comprises a multiplicity of hydrogen gas detector elements eachof which (i) is arranged for exposure to a specific individual locus ofthe extended area region and (ii) employs an optical filter comprising arare earth metal thin film that exhibits a detectable change in opticaltransmissivity when the rare earth metal thin film is contacted withhydrogen at such locus.

[0025] A further aspect of the invention relates to a method offabricating a hydrogen gas detector, comprising:

[0026] providing a source of luminous and thermal energy including anoutput surface for emitting light and thermal energy;

[0027] depositing on the output surface an optical filter comprising arare earth metal thin film that responds to contact with hydrogen byexhibiting a detectable change of optical transmissivity;

[0028] positioning a light detector in light-sensing proximity to thesource of luminous and thermal energy, whereby a change in opticaltransmissivity of the rare earth metal thin film in exposure to hydrogengas is detected as a change in luminous energy flux impinging on thedetector, and

[0029] outputting a signal indicative of the change in luminous energyflux.

[0030] Other aspects, features and embodiments of the invention will bemore fully apparent from the ensuing disclosure and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1A is a schematic representation of the present inventionaccording to one embodiment, in the absence of hydrogen gas.

[0032]FIG. 1B is a schematic representation of the present inventionaccording to a second embodiment, in the absence of hydrogen gas.

[0033]FIG. 2 is a schematic representation of the present inventionaccording to one embodiment, in the presence of hydrogen gas.

[0034]FIG. 3 is a schematic representation of the present inventionaccording to a second embodiment, in the absence of hydrogen gas.

[0035]FIG. 4 is a schematic representation of the present inventionaccording to a second embodiment, in a low concentration of hydrogengas.

[0036]FIG. 5 is a schematic representation of the present inventionaccording to a second embodiment, in a high concentration of hydrogengas.

[0037]FIG. 6 is a graph depicting a typical optical response curve forthe light transmitted through the end of the coated lamp elements in thepresence of hydrogen, according to one embodiment of the presentinvention.

[0038]FIG. 7 is a graph depicting the time response curves of hydrogendetectors according to one embodiment of the present invention, indiffering concentrations of hydrogen.

[0039]FIG. 8 is a graph depicting the maximum response of one embodimentof the present invention as a function of hydrogen concentration.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF

[0040] The disclosure of U.S. patent application Ser. No. 09/042,698filed Mar. 17, 1998 in the names of Gutam Bandari and Thomas H. Baum for“Hydrogen Sensor Utilizing Rare Earth Metal Thin Film Detection Element”is hereby incorporated herein by reference in its entirety.

[0041] The present invention utilizes a light/heat source, an opticaldetector, and an optical barrier between the source and detector. Theoptical barrier responds to the presence of hydrogen and transitionsfrom a first optical state, e.g., a state of optical opacity, in theabsence of hydrogen to a second and different optical state, e.g., astate of optical translucency/transparency in the presence of hydrogen.

[0042] In general, the optical barrier may comprise any suitablematerial whose optical transmissivity characteristics arehydrogen-dependent, being in one of different optical states dependenton the presence or absence of hydrogen. By way of specific example, thehydrogen-sensitive optically variable material may comprise a rare earthmetal thin film of the type disclosed in the aforementioned U.S. patentapplication Ser. No. 09/042,698, optionally overcastted with ahydrogen-permeable protective layer such as palladium. The opticallyvariable character of rare earth metal films is based on the reversible,hydrogen-induced transition from the metallic dihydride compound to thesemiconducting trihydride compound. For example, in the case of yttrium,this transition is represented by the following equation involvingyttrium:

[0043] wherein the dihydride compound is optically opaque in thin filmform, and the trihydride is optically transparent in thin film form.

[0044] As used herein, the term “rare earth metal thin films” will beunderstood as broadly referring to thin films (e.g., films having athickness of less than about 1,000 microns) of rare earth metals of thePeriodic Table having the atomic numbers of 51 to 71 inclusive, whichare capable of existing in divalent as well as trivalent hydride forms,as well as yttrium. Of the rare earth metals in the lanthanide series ofthe Periodic Table, lanthanum is particularly preferred. While yttriumis not technically a rare earth metal, it is included because thin filmsof yttrium exhibit properties similar to those of thin films of rareearth metals (having the atomic numbers of 51 to 71 inclusive) of thePeriodic Table.

[0045] In the use of rare earth metal thin films in the practice of theinvention for hydrogen detectors, in applications in which the thin filmwill or may encounter oxidizing species in the environment beingmonitored for hydrogen, such as oxygen, moisture (relative humidity),nitrogen oxides, carbon oxides, etc., it is advantageous to coat orencapsulate the rare earth metal thin film with a hydrogen-permeableprotective material that prevents such oxidizing species from contactingthe rare earth metal thin film.

[0046] The protective material may for example absorb oxygen and allowdiffusion of hydrogen through the protective material to the rare earthmetal thin film. Alternatively, the protective material may beimpermeable to oxygen and/or other oxidizing species.

[0047] The protective material when present as an overlayer coating orencapsulate should be continuous and atomically dense in order toprovide an effective barrier against oxidation. The thickness of theoverlayer may be readily selected to minimize oxygen permeation whilemaximizing the response of the rare earth metal thin film to hydrogen.

[0048] In one embodiment of the present invention in which a protectivematerial overlayer is employed, the overlayer may be formed of a noblemetal such as Pd, Pt, or Ir, or alloys or combinations thereof with oneanother or with other metal species. Particularly useful alloys for suchprotective material overlays include Pd—Ag and Pd—Ni.

[0049] Any suitable forming or deposition process may form the rareearth metal thin film. For example, the rare earth metal thin film maybe formed by chemical vapor deposition, physical vapor deposition,solution deposition, electroplating, electroless plating, sputtering,pulsed laser deposition or any other suitable technique or methodologyfor formation or deposition thereof. The rare earth metal film may beformed on any suitable substrate, e.g., silicon, silicon oxide, siliconcarbide, alumina, vitreous or ceramic materials, etc.

[0050] A particularly preferred technique for forming the rare earthmetal thin film in the broad practice of the present invention ischemical vapor deposition (CVD). Due to its high throughput and lowcost, the CVD process is advantageous in fabricating sensor devices ofthe present invention in an efficient and economic manner. The abilityof CVD to conformably coat substrates provides further benefit in thedeposition of a protective overlayer material, in instances where thehydrogen sensor of the invention is fabricated with such a barriermaterial for preventing the reaction of the rare earth metal withoxidizing species in the environment being monitored for hydrogen.

[0051] The CVD process may utilize bubbler delivery or liquid deliverywith subsequent flash vaporization, using a suitable rare earth metalprecursor or source compound, to generate a precursor vapor which istransported to a heated substrate for decomposition to form the desiredrare earth metal film. Such precursors must be robust and volatile atthe temperature of vaporization, yet they must decompose cleanly andefficiently on the substrate.

[0052] Suitable precursors may be readily determined within the skill ofthe art by screening techniques conventionally used in the art of CVDformation of thin films, including thermogravimetric analysis (TGA) anddifferential scanning calorimetry (DSC) analysis.

[0053] The light/heat source used in the hydrogen detector of thepresent invention may comprise any suitable source of both luminescentand thermal energy, either together or separately. The light source mayfor example comprise an incandescent bulb, LED, fluorescent tube,electroluminescent lamp, or laser. The heat source may for examplecomprise a resistive wire (including the filament of an incandescentbulb); an exothermic chemical reaction; ultrasonic, acoustic, microwave,laser, or other directed radiation; or the like.

[0054] The optical detector used in the practice of the presentinvention may likewise comprise any suitable detector as known in theart, such as a photodiode, scintillation detector, photomultiplier tube,etc. The optical detector can be of structural or electrical/electroniccharacter, or the optical detector may otherwise comprise the human eye.

[0055] In one preferred embodiment of the present invention, thelight/heat source comprises a miniature incandescent light bulb, and thehydrogen-sensitive optical barrier is a rare earth metal thin filmcoating deposited directly on the surface of bulb.

[0056] In the absence of hydrogen, light generated from the bulbfilament is blocked from the detector by the opaque rare earth metalthin film. In the presence of hydrogen, the coating partially transformsto a transparent trihydride state, becoming translucent in overallcharacter and thus light-transmissive in effect. Light can thus passthrough the coated bulb and reach the detector, so that the occurrenceof light transmission is indicative of the presence of hydrogen.

[0057] The optical detector may be electrical or electronic incharacter, and may be constructed and arranged to provide a suitableoutput. Further, by appropriate calibration, and correspondinglysensitive detector componentry, the concentration of hydrogen in theenvironment being monitored can be determined quantitatively, andoutputted to a use of the detector.

[0058] In the embodiment described above, wherein a rare earth metalthin film is coated on a light source concomitantly with the productionof light producing heat, the thermal energy generated by the lightsource will heat the rare earth metal thin film coating, therebyminimizing the “recovery” time necessary for the rare earth metal thinfilm to “reverse-transition” from the transparent tri-hydride formincident to hydrogen exposure, to an opaque dihydride upon removal ofhydrogen gas and/or the detector from the environment in which thedetector was initially exposed to hydrogen.

[0059] In the above-described embodiment, featuring a hydrogen-sensitivefilm on a thermally emissive light source, the amount of lighttransmitted will be, in general, a function of the concentration ofhydrogen present. This feature permits the detector of the presentinvention to be employed as a rudimentary hydrogen concentrationmeasurement device, which as mentioned hereinabove can be calibrated toprovide quantitative information about the hydrogen present in thegaseous environment being monitored. Alternatively, thisconcentration-dependent character of the optical transmissivity of thethin film material allows particular detectors to be engineered oradjusted for sensitivity to specific concentrations of hydrogen.

[0060] The present invention thus contemplates a hydrogen gas detectorfor detection of hydrogen gas in a gaseous environment. The detectorsimply and conveniently comprises a light/heat source, an opticaldetector, and an optical barrier between the source and detector. Theoptical barrier responds to the presence of hydrogen by responsivelychanging from a first optical state to a different second optical state,whereby transmission of light from the light/heat source through theoptical barrier is altered by the presence of hydrogen. The resultantaltered transmission is sensed by the optical detector to provide anindication of the presence of hydrogen gas in the gaseous environment.

[0061] The first optical state suitably comprises a state of opticalopacity of the optical barrier, while the second optical state comprisesa state of optical non-opacity of the optical barrier, e.g., a state oftranslucency/transparency of the optical barrier. The optical barriersuitably comprises a rare earth metal thin film, such as an yttrium thinfilm, optionally overlaid by a protective film that is permeable tohydrogen gas such as a palladium film.

[0062] The hydrogen gas detector of the invention may be usefullyembodied as a unitary portable article, e.g., comprising a power supplysuch as a battery. The hydrogen gas detector may suitably incorporate anoutput module operatively coupled to the optical detector, and arrangedto provide an output alarm indicative of the presence or concentrationof hydrogen gas in the gaseous environment, e.g., a visual, audible ortactile alarm.

[0063] The light/heat source of the detector in a simple embodimentcomprises a lamp element providing heat output incident to thegeneration of light, such as an incandescent lamp. The incandescent lampmay be formed in a conventional manner as including a light-transmissivebulb, on an exterior surface of which is coated a rare earth metal thinfilm as the optical barrier. Such hydrogen gas detector may be of a verycompact form, e.g., of a hand-held size and character, wherein thelight/heat source comprises a lamp element providing heat outputincident to the generation of light, with an yttrium thin film coated onthe lamp element as the optical barrier, and a protective palladium filmcoated on the yttrium thin film.

[0064] The hydrogen gas detector of the invention may be operativelycoupled with corresponding hydrogen gas detectors to form an extendedarea monitoring system for detection of hydrogen gas in the gaseousenvironment of such extended area. For example, a hydrogen detectionsystem for monitoring an extended or remote area region for theincursion or generation of hydrogen therein, may suitably comprise amultiplicity of hydrogen gas detector elements each of which (i) isarranged for exposure to a specific individual locus of the extendedarea region and (ii) employs an optical filter comprising a rare earthmetal thin film that exhibits a detectable change in opticaltransmissivity when the rare earth metal thin film is contacted withhydrogen at such locus.

[0065] The hydrogen gas detector of the invention may therefore includein an illustrative embodiment the following components: (i) a lightsource; (ii) a thermal energy source; (iii) an optical filter having anoptical transmissivity responsive to the presence and concentration ofhydrogen gas in an ambient environment to which the optical filter isexposed, such optical filter being disposed in proximity to the lightsource so that the optical filter is illuminated with light from thelight source, and being operatively coupled to the thermal source sothat the optical filter is heated by the thermal source; and (iv) alight detector generating an output signal, wherein the state of theoutput signal is proportional to the intensity of light impinging on thelight detector. The light detector is suitably disposed in light-sensingrelationship to the optical filter, whereby light from the light sourcepassing through the optical filter impinges on the light detector andgenerates the output signal as a indication of the presence and/orconcentration of hydrogen gas in the ambient environment.

[0066] In the hydrogen gas detector of the invention, the source ofluminous energy may be a light-generating element selected from amongincandescent bulbs, light emitting diodes, fluorescent lamps,electroluminescent lamps, and optical lasers. The thermal energy sourcemay be a heat-generating element such as an incandescent bulb, resistivewire, exothermic chemical reaction, ultrasonic radiation, acousticradiation, microwave radiation, or laser radiation.

[0067] The light source and the thermal energy source may comprise asame element, or alternatively the light source and the thermal energysource may comprise different elements.

[0068] The light detector may comprise any suitable light detectionelements, e.g., photodiodes, avalanche photodiodes, phototubes,photomultiplier tubes, microchannel plates, solar cells, imageintensifiers, photoconductor detectors, charge-coupled devices, orcombinations or arrays thereof.

[0069] The optical filter preferably comprises a rare earth metal thinfilm deposited on an optical output surface of the light source. Therare earth metal thin film may comprise a rare earth metal componentselected from the group consisting of trivalent rare earth metalsreactive with hydrogen to form both metal dihydride and metal trihydridereaction products, wherein the metal dihydride and metal trihydridereaction products have differing optical transmissivity.

[0070] The rare earth metal thin film suitably comprises at least onemetal selected from the group consisting of:

[0071] scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, actinium, thorium,protactinium, uranium, neptunium, plutonium, americium, curium,berkelium, californium, einsteinium, fermium, mendelevium, nobelium, andlawrencium,

[0072] alloys thereof, and

[0073] alloys containing one or more of such metals alloyed with analloying component selected from the group consisting of magnesium,calcium, barium, strontium, cobalt and iridium.

[0074] Most preferably, the rare earth metal thin film comprisesyttrium.

[0075] The rare earth metal thin film may optionally be overlaid by ahydrogen-permeable material comprising a metal such as Pd, Pt, Ir, Ag,Au, Ni, Co, or an alloy thereof. The rare earth metal thin film may alsooptionally be overlaid by a hydrogen-permeable material that is dopedwith a dopant, e.g., Mg, Ca, Al, Ir, Ni or Co.

[0076] Preferred overlay metals include palladium, platinum, andiridium.

[0077] The hydrogen gas detector may be fabricated by the followingsequence of steps:

[0078] First, a source of luminous and thermal energy is provided. Suchsource includes an output surface for emitting light and thermal energy.Next, an optical filter is deposited on the output surface. The opticalfilter comprises a rare earth metal thin film that responds to contactwith hydrogen by exhibiting a detectable change of opticaltransmissivity.

[0079] Next, a light detector is positioned in light-sensing proximityto the source of luminous and thermal energy, whereby a change inoptical transmissivity of the rare earth metal thin film in exposure tohydrogen gas is detected as a change in luminous energy flux impingingon the detector, to output a signal indicative of the change in luminousenergy flux. Such output may be of any suitable type, e.g., visualoutputs, optical outputs, tactile outputs, electrical outputs and/orauditory outputs.

[0080] The rare earth metal thin film may be formed on the outputsurface of the source of luminous and thermal energy, by a techniquesuch as physical vapor deposition, chemical vapor deposition,sputtering, solution deposition, focused ion beam deposition, pulsedlaser deposition, electrolytic plating, or electroless plating.

[0081] In one embodiment, the rare earth metal thin film is formed onthe substrate by chemical vapor deposition using an organometallicprecursor that thermally decomposes to the metal hydride or elementalmetal in a reducing environment of hydrogen.

[0082] The rare earth metal thin film in the practice of the inventionadvantageously comprises at least one metal selected from among thefollowing:

[0083] scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, actinium, thorium,protactinium, uranium, neptunium, plutonium, americium, curium,berkelium, californium, einsteinium, fermium, mendelevium, nobelium, andlawrencium,

[0084] alloys thereof, and

[0085] alloys containing one or more of such metals alloyed with analloying component selected from the group consisting of magnesium,calcium, barium, strontium, cobalt and iridium.

[0086] The rare earth metal thin film suitably comprises a rare earthmetal component selected from the group consisting of trivalent rareearth metals reactive with hydrogen to form both metal dihydride andmetal trihydride reaction products, wherein the metal dihydride andmetal trihydride reaction products have differing opticaltransmissivity, and wherein transitions between the metal dihydride andmetal trihydride reaction products are caused by the presence or absenceof hydrogen gas contacting the rare earth metal thin film, e.g., a rareearth metal thin film overlaid by a hydrogen-permeable material, such asPd, Pt, Ir, Ag, Au, Ni, Co, or alloys thereof, and optionally doped witha dopant selected from the group consisting of Mg, Ca, Al, Ir, Ni andCo.

[0087] In one embodiment, the rare earth metal thin film is formed of ametal such as lanthanum or yttrium, and the rare earth metal thin filmis formed on the output surface of the light/heat source by CVDutilizing corresponding precursors, e.g.,tris(cyclopentadienyl)lanthanum, tris(cyclopentadienyl)yttrium,β-diiminate complexes of lanthanum, β-diiminate complexes of yttrium;lanthanum amides, and yttrium amides.

[0088] When the rare earth metal thin film comprises lanthanum, the rareearth metal thin film preferably is formed on the substrate by CVDutilizing a precursor such as La(NR₂)₃, La(NR₂)₃.L, La(R)₃ or La(R)₃.Lwherein R is C₁ to C₈ alkyl or C₁ to C₈ aryl and L is a Lewis baseligand selected from the group consisting of amines, aryls and arylamines, more specifically, NH3, primary amines, secondary amines,tertiary amines, polyamines, and more specifically, pyridine,methylamine, dimethylamine trimethylamine, dimethylethylamine,N,N,N′,N′-tetramethylethylenediamine and N,N,N′,N′,N″-pentamethyldiethylenetriamine and such Lanthanum precursor may bedeposited in the presence of a reducing and/or inert gas, e.g. NH₃,NH₃/H₂ or NH₃/N₂.

[0089] When the rare earth metal thin film comprises yttrium, the rareearth metal thin film may be formed on the substrate by CVD utilizing aprecursor, such as Y(NSiR′₃)₃, wherein each of R′ may be same ordifferent and are independently selected from the group consisting of C₁to C₈ alkyl or C₁ to C₈ aryl.

[0090] When palladium is utilized as an overlayer material, theoverlayer may be formed by chemical vapor deposition on the rare earthmetal thin film, using a palladium precursor such as Pd(hfac)₂,Pd(allyl)₂, Pd(allyl)(hfac), Pd(methylallyl)(hfac), CpPd(allyl) orCOD.Pd(Me)₂.

[0091] An illustrative embodiment of the present invention is depictedin FIGS. 1 and 2, wherein like components are correspondingly numberedfor ease of reference.

[0092] Referring to FIGS. 1A and 1B, light bulb 10 comprisingincandescent filament 16 has deposited thereon a rare earth metal thinfilm layer 12, preferably comprising a trivalent rare earth metal suchas yttrium, that is reversibly reactive with hydrogen to form both metaldihydride and metal trihydride reaction products. Over the rare earthmetal thin film layer 12 is deposited a protective layer 14, comprisinga suitable material, such as for example Pd, Pt, Ir, Ag, Au, Ni, Co, oralloys thereof, and most preferably comprising palladium. In the absenceof hydrogen in the ambient environment to which the bulb is exposed, therare earth metal thin film 12 is in a metallic, optically reflectivedihydride state. Light from filament 16 is attenuated by the dihydridestate of rare earth metal thin film layer 12 and thus only a portion ofit reaches photo-detector 18.

[0093] The light bulb 10 is connected by power supply transmission wire22 to a power supply 20. The power supply 20 is joined by power supplywire 24 to the photo-detector 18, to provide power to the latter. Thephoto-detector 18 is joined by output signal transmission wire 26 tooutput module 28, which may provide a visual, audible, tactile or otheralarm indicative of the presence of hydrogen in the gaseous environmentbeing monitored, or which may provide a quantitative output correlativeof the concentration of hydrogen in the environment being monitored.

[0094]FIG. 2 shows hydrogen (denoted symbolically by multiple “H”s inthe environs of the detector) present in the atmosphere to which coatedlight bulb 10 is exposed. The hydrogen diffuses through protective layer14 and reacts with rare earth metal thin film layer 12. This causesportions of rare earth metal thin film 12 to transition to asemiconducting, optically transparent trihydride state. Some light fromfilament 16 thereby passes through the translucent rare earth thin filmlayer 12 to impinge on photo-detector 18, generating a signaltransmitted in line 26 to the output module 28, corresponding to thepresence and intensity of incident light.

[0095] In FIG. 1A, filament 16 of coated light bulb 10 is additionally aheat source, elevating the temperature of rare earth thin film 12. InFIG. 1B, heat-generating element 17 is depicted as a resistive element.However, heat-generating element 17 may comprise incandescent bulbs,resistive element 17 may comprise incandescent bulbs, resistive wires,exothermic chemical reactions, ultrasonic radiation, acoustic radiation,microwave radiation, laser radiation or other such heat-generatingradiation, laser radiation or other such heat-generating elements asknown to those skilled in the art. The transition of rare earth thinfilm 12 from reflective dihydride to transparent trihydride state andback, in response to the absence or presence, respectively, of hydrogenoccurs much more rapidly at elevated temperatures. This reduces both theresponse time of the detector in the presence of hydrogen and itsrecovery to the opaque “null state” in the absence of hydrogen.

[0096] An alternative illustrative embodiment of the present inventionis depicted in FIGS. 3, 4, and 5, wherein like components arecorrespondingly numbered with each figure and with FIGS. 1 and 2, forease of reference. This embodiment demonstrates one manner in whichdifferent bulb coatings and multiple detectors may be employed toincrease the dynamic range of the hydrogen gas detector.

[0097]FIG. 3 depicts light bulb 10 comprising incandescent filament 16and having deposited thereon a rare earth metal thin film layer 12,preferably comprising a trivalent rare earth metal such as yttrium, thatis reversibly reactive with hydrogen to form both metal dihydride andmetal trihydride reaction products. Over the rare earth metal thin filmlayer 12, on approximately half of the optical output surface of thebulb, is deposited a protective layer 13, comprising a suitable materialwith a relatively high permeability to hydrogen, such as for example Pd,Pt, Ir, Mg, Ca, Ag, Au, Ni, Co, or alloys thereof, and most preferablycomprising palladium. Over the rare earth metal thin film layer 12, onapproximately the opposing half of the optical output surface of thebulb from protective layer 13, is deposited a protective layer 15,comprising a suitable material with a relatively lower permeability tohydrogen as compared to protective layer 13, such as for example Pt, Ir,Mg, Ca, Ag, Au, Ni, Co, or alloys thereof, and most preferablycomprising Iridium. Arranged in light-receiving relationship with theside of coated bulb 10 corresponding to high hydrogen permeabilityprotective layer 13 is photo-detector 18A. Similarly, photo-detector 18Bis arranged in light-receiving relationship with the side of coated bulb10 corresponding to low hydrogen permeability protective layer 15. Inthe absence of hydrogen in the ambient environment to which the bulb isexposed, the rare earth metal thin film 12 is in a metallic, opticallyreflective dihydride state. Light from filament 16 is blocked by thereflective dihydride state of rare earth metal thin film layer 12 andthus does not reach either photo-detector 18A or 18B.

[0098] The light bulb 10 is connected by power supply transmission wire22 to a power supply 20. The power supply 20 is joined by power supplywire 24A to the photo-detector 18A, and by power supply wire 24B to thephoto-detector 18B, to provide power to each photo-detector. Thephoto-detector 18A is joined by output signal transmission wire 26A tooutput module 28A, which may provide a visual, audible, tactile or otheralarm indicative of the presence of hydrogen in the gaseous environmentbeing monitored at a first concentration level corresponding to therelatively high hydrogen permeability of protective layer 13. Similarly,photo-detector 18B is joined by output signal transmission wire 26B tooutput module 28B, which may provide a visual, audible, tactile or otheralarm indicative of the presence of hydrogen in the gaseous environmentbeing monitored at a second concentration level corresponding to therelatively low hydrogen permeability of protective layer 15.

[0099]FIG. 4 shows a relatively low concentration of hydrogen (denotedsymbolically by several “H”s in the environs of the detector) present inthe atmosphere to which coated light bulb 10 is exposed. The hydrogendiffuses through high hydrogen permeability protective layer 13 andreacts with the half of rare earth metal thin film layer 12 thatunderlies it. This causes portions of that half of rare earth metal thinfilm 12 to transition to a semiconducting, optically transparenttrihydride state. Some light from filament 16 thereby passes throughthat half of the translucent rare earth thin film layer 12 to impinge onphoto-detector 18A, generating a signal transmitted in line 26A to theoutput module 28A, corresponding to the presence and intensity ofincident light, and indicative of presence of hydrogen in theenvironment of bulb 10 of at least a first characteristic concentration.The relatively low concentration of hydrogen in the environment of bulb10, together with the relatively low hydrogen permeability of protectivelayer 15, is insufficient to react enough hydrogen with the half of rareearth metal thin film layer 12 which underlies protective layer 15 totrigger a similar transition. This half of rare earth metal thin filmlayer 12 remains in a metallic, optically reflective dihydride state,and light from filament 16 is blocked from reaching photo-detector 18B.Thus, no signal is transmitted in line 26B to the output module 28B,indicating that the hydrogen gas present in the environment of bulb 10(as indicated by output module 28A) is below a second characteristicconcentration.

[0100]FIG. 5 shows a relatively high concentration of hydrogen (denotedsymbolically by many “H”s in the environs of the detector) present inthe atmosphere to which coated light bulb 10 is exposed. The hydrogendiffuses through both the high hydrogen permeability protective layer 13and the low hydrogen permeability protective layer 15, and reacts withboth halves of rare earth metal thin film layer 12. This causes portionsof both halves of rare earth metal thin film 12 to transition to asemiconducting, optically transparent trihydride state. Some light fromfilament 16 thereby passes through both halves of the translucent rareearth thin film layer 12 to impinge on both photo-detector 18A andphoto-detector 18B. Corresponding signals are transmitted in line 26A tothe output module 28A, and in line 26B to the output module 28B,respectively. The visual, audible, tactile or other alarm present atboth output modules 28A and 28B indicates the presence of hydrogen gasin the environment of bulb 10 of at least a second characteristicconcentration. Such a two-step hydrogen gas detector alarm may beadvantageously employed where, for example, the low concentration alarmfrom output module 28A may serve as a warning, indicating investigationis warranted, while the high concentration alarm from output module 28Bmay indicate an unacceptable safety hazard, necessitating evacuation.

[0101] In yet another alternative embodiment, the dynamic range ofhydrogen gas detection quantization may be expanded by employing aplurality of incandescent bulbs, and associated alarm-triggeringphoto-detectors, wherein each bulb is coated with a rare earth metalthin film layer, preferably comprising a trivalent rare earth metal suchas yttrium, that is reversibly reactive with hydrogen to form both metaldihydride and metal trihydride reaction products. The rare earth metalthin film layer on each bulb may be overlaid by a protective layer,comprising a suitable material, such as for example Pd, Pt, Ir, Mg, Ca,Ag, Au, Ni, Co, or alloys thereof, wherein the hydrogen permeability ofthe protective layer on each bulb is selected and/or engineered totrigger a transition in the underlying trivalent rare earth metal thinfilm from a metallic, optically reflective dihydride state to asemiconducting, optically transparent trihydride state at or above acertain characteristic concentration of hydrogen gas in the environmentof the coated bulb. Alternatively, or additionally, the transitioncharacteristics of the various rare earth metal thin films in the arraymay be altered by operation of the bulbs at different light intensitiesand/or operating temperatures, and/or by altering the switchingthreshold of the photo-detectors. Within the broad practice of thisembodiment of the present invention, the optimal number ofbulb/photo-detector pairs, the characteristics of the variouslyengineered protective layers, the operating intensity of theincandescent bulbs, the switching threshold of the associatedphoto-detectors, and other system parameters may vary widely, dependingon the application, and may be determined by one of ordinary skill inthe art without undue experimentation.

[0102] In all of the above embodiments of the present invention, it isdesirable to improve the speed and responsive of the rare earth metalthin films whose physical property changes in the presence of hydrogengas are exploited to effect hydrogen gas detection. Applicants havediscovered that the response time is strongly dependent on grain size,and that for a given grain size, an increase in surface roughnessresults in an increased speed of response, as compared to deposition onun-roughened substrates. Four methods of increasing the response time ofthe rare earth metal thin films by increasing the surface morphologyroughness have been identified, and comprise alternate embodiments ofthe present invention. These four methods are: mechanical roughening;chemical roughening; deposition of roughened inorganic underlayers; anddeposition of porous polymer underlayers, based on interpenetratingpolymer networks.

Mechanical Roughening

[0103] There are several methods of mechanical roughening that can beused on substrates to increase the surface roughness of subsequentlydeposited rare earth noble metal bi-layer thin films. The first methoduses abrasives, i.e. sand paper, metal files, polishing pads andpolishing pastes or compounds. The substrate can be roughened to thedesired finish through the choice of material, and choice of the grade,grit, or particle size. The abrasive can be a material such as Al₂O₃,SiC, or diamond based or any other suitably hard material. Anothermethod of mechanical roughening is the use of a bead blasting technique,where again the final finish can be determined through the choice of thebead blasting material. These methods can be done in either a dry or awet process. In wet processing, a number of fluids may be considered,such as water, mineral oil, or organic solvents.

Chemical Roughening

[0104] Chemical Roughening of the substrate can be achieved by dippingin an acid, such as HF, dilute HF, or buffered HF solution. In thesemethods, the amount of roughening will be determined by the type andconcentration of the etchant, as well as the composition and initialroughness of the substrate. Chemical roughening may be appliedsequentially to mechanical roughening to produce a desired morphology.

Deposition of Highly Exfoliated or Porous Inorganic Underlayers

[0105] This method is primarily founded on sol gel deposition ofmorphological rough SiO₂ or Al₂O₃ thin film layers. One method forachieving this is through the coating of the substrate with aTEOS/alcohol/acidified aqueous solution. The coating method controlsthickness and uniformity of the film. The substrate may be dip coated,spray coated, or spin cast or lyophilized. The curing of this coatingresults in a porous or high surface area oxide thin film. The chemistryutilized in the sol-gel precursor stage and the drying techniquesemployed after the sol-gel is cast determine the porosity (microscopicto mesoporous), surface area (upwards of 900 m²/g) and roughness of thethin film (Jeffrey Brinker, George Scherer, Sol-Gel Science: The Physicsand Chemistry of Sol-Gel Processing, Academic Press, San Diego, Calif.(1990)). Subsequent deposition of rare earth noble metal bi-layer thinfilms onto the high area sol-gel will create a hydrogen sensitive devicethat is easily accessed by the gas of interest. The large diffusioncoefficient of gases such as H₂ (0.84 cm²/s at 295 K, 1 bar) insure thatgaseous diffusion will not limit the response of the sensor.

Deposition of Porous Polymer Underlayers

[0106] Deposition of porous polymer underlayers, based on morphologicalcontrol of interpenetrating, phase segregated, or copolymer polymersalso allows one to achieve desirable high surface area structures fordeposition of the active bi-metal layer.

[0107] Phase segregation of polymer mixtures from a common solvent hasbeen used in anti-reflective coating methods (Walheim, S.,Macromolecules, 30, pp.4995-5003 (1997)). A non-miscible polymer pair,such as PVC and polybutadiene, may be solvated at up to 5% weight in acommon solvent, such as THF, and cast onto a surface to allow solventevaporation upon which segregation of the polymer pair occurs.Dissolution of one of the polymer pair, polybutadiene, in a non-commonsolvent, hexane, leaves a porous PVC structure. Solvent, polymer pair,and molecular weight of the polymers all contribute to control themorphology.

[0108] Interpenetrating polymer networks from two part monomer or onepart monomer-one part polymer systems also allow a controlled morphologyto be synthesized which may subsequently be used as a high surface areathin film polymer film. For example, a methacrylate monomer and TEOS maybe mixed in a common solvent, cast as a homogeneous thin film, andsubsequently polymerized. Subsequent removal of one of theinterpenetrating polymers will leave a highly porous film of the otherpolymer. For example, SiO₂, the product of TEOS polymerization, isreadily removed with ½% anhydrous HF which would leave behind apolymethacrylate thin film network.

[0109] Nearly monodisperse copolymers manufactured from anionicpolymerization of monomers whose polymers are non-miscible also exhibitphase segregation at the microscopic level. The radius of gyration ofthe polymer segments, solvent choice, and interfacial free energy of thebi-phase system determine the domain size and shape of themicrostructure. A well known example uses a styrene-isoprene copolymerto produce regular arrays of anisotropically oriented tubes withnanometer dimensions (Maurice Morton, Anionic Polymerization: PrinciplesAnd Practice, pp211, Academic Press, New York, N.Y. (1993)). Depositionof a bimetallic layer at high aspect ratios may be difficult in whichcase carbon nanotube research has demonstrated that small diameterspossessing high surface tensions may promote capillary wetting of themicroscopic tubes walls by a chemical route (M. Terrones et. al., MRSBulletin, 24, 8, pp. 44, (1999)). For example, deposition of a CVDprecursor from the bulk solvent may wet the walls of the nanoporouspolymer film. The metal from the CVD precursor would subsequently bereduced followed by the second metal CVD precursor that would form thetop layer of the bi-metal active material.

[0110] In still another embodiment of the present invention, the lightand heat source for the hydrogen gas detector, as well as optionally theoptical input to the photo-detector, may comprise fiber optics, quartzrods, or other optical waveguides. The trivalent rare earth metal thinfilm may, in such embodiment, be deposited directly on the opticaloutput surface of the optical waveguide, or alternatively on a surfacecoupled in light-receiving relationship with the waveguide. Hydrogen gasdetectors according to this embodiment of the present invention presentsignificant advantages, including the ability to be conformably routedto many places where installation of incandescent bulbs would beimpractical due to size or space considerations, or would presentunacceptable safety risks.

[0111] To obtain satisfactorily rapid response/recovery time, thetrivalent rare earth metal thin film must be heated. One means ofaccomplishing this in a fiber optics waveguide application is toconstruct a layered substrate at the optical output surface that wouldabsorb optical energy at one wavelength, particularly in the IR region,but would transmit optical energy at a different wavelength (thewavelength of the associated detector). Thus, one high-intensitywavelength is used to heat the optical barrier, and a second wavelengthprovides the signal, or probe, directed at the optical detector.

[0112] One method of forming such a layered substrate, and oneembodiment of the present invention, is through sol gel deposition ofmorphological rough SiO₂ or Al₂O₃ thin film layers as described above.Subsequent deposition of rare earth noble metal bi-layer thin films ontothe sol-gel will create unitary light/heat source and optical barrierwith selective light transmission properties dependent upon theconcentration of hydrogen gas in the region of the thin film.

[0113] Another way to utilize one wavelength of a polychromatic lightfor heating, and another as a signal/probe, and a separate embodiment ofthe present invention, is to mix a wavelength-specific dye into the solgel solution before deposition on the SiO₂ or Al₂O₃ substrate. This dyewill absorb the heating wavelength, while permitting the passage of theprobing wavelength(s).

[0114] The hydrogen gas detectors of the present invention are small,inexpensive, require low power, and are easily maintained. A fullyfunctional detector unit requires only a coated incandescent bulb asdescribed above, a photo-detector, an appropriate power source (that maycomprise batteries, making the entire unit self-contained and portable),and a housing that shuts out ambient light but allows for the flow ofgas over and around the coated bulb.

[0115] In inherently dark environments, such as the interior of pipes,ducts, and similar gas-flow passageways, or interior to normally sealedequipment, no ambient light-blocking housing is necessary.

[0116] Alternatively, an ambient light-blocking housing may be avoidedby matching the wavelength of light emanating from the optical sourcewith a narrow-band optical detector, the operative wavelength beingselected as one not normally included within the ambient lightingconditions at a given location. This may be necessary in certainapplications where the coated bulb of the detector must be introduceddirectly into a gaseous stream, or where the gaseous flow is of such lowvolume that an ambient light-blocking housing will degrade detectionperformance.

[0117] Multiple optical hydrogen gas detector units may be combined tomonitor the presence of hydrogen gas over a large area. Individualunit's detector outputs may all be transmitted, via wired, radio, oroptical communications media, to a central processing unit forcomprehensive monitoring of an extended area, whereby the output of eachsensor is mapped to its individual locus. Such a system could monitorthe spread of hydrogen gas throughout an area over time, or calculatesafe pathways of egress, selectively activating emergency exit indiciaand/or denying access to the areas in which hydrogen is detected.

[0118] Alternatively, the output of each individual optical hydrogen gasdetector could be connected (i.e., via a relay or transistor) in seriesarrangement with all other detectors. In this configuration, hydrogengas detected by any one detector anywhere within the monitored areawould result in triggering of, e.g., an alarm or shut-down signal.

[0119] Various portable embodiments of the hydrogen gas detector unitsof the present invention are readily fabricated and utilized. Suchportable units may as indicated hereinearlier be battery-powered, asregards the coated bulb, photo-detector, and output module (comprisingan output component such as an LED or piezoelectric audible alarm). Theportability of such hydrogen gas detector units is a critical safetyfeature for personnel who must work in environments where hydrogen gasis stored, moved, or utilized.

[0120] Portable hydrogen gas detectors according to this embodiment ofthe present invention could be incorporated into other equipment, withthe potential added efficiencies of sharing a power supply or otherresources. Examples include lighted hard hats, radio communicationdevices, battery-powered hand tools, portable computing devices,electronic badges and “smart cards” used for identification andsecurity, etc.

[0121] The invention will be further understood and illustrated by thefollowing non-limiting example.

EXAMPLE

[0122] A series of hydrogen detectors of the generally typeschematically depicted in FIGS. 1 and 2 was fabricated utilizing as thethermally emissive light source in each detector a miniatureincandescent lamp (Model #8-374) commercially available from ChicagoMiniature Lamp, Inc, (Hackensack, N.J.). These lamp elements havefocusing lens-tips, and are rated at 2.5 volts and 0.350 amps. The lampelements were held with their tips held pointing down in a sample holderand placed in an e-beam evaporator. A first layer of 100 nanometers ofgadolinium magnesium alloy was deposited, followed by a layer of 15 nmof palladium. The gadolinium magnesium alloy was created by alternatelydepositing of gadolinium and then magnesium layers. Two layers ofmagnesium (15 nm) were layered between 3 layers of gadolinium (23.3 nm),such that the sequence was Gd/Mg/Gd/Mg/Gd, and the total compositionratio was Gd:Mg 70:30 by volume. The deposition was carried out at 300°C., which results in the mixing of the Gd/Mg layers to form an alloy.The bulb was placed in a hydrogen gas test-cell, with appropriateelectrical feed-throughs, and powered with a DC power supply. The lightoutput was measured over the spectral range of 650-1100 nm with a fiberoptic spectrometer.

[0123]FIG. 6 shows a typical optical response curve for the lighttransmitted through the end of the coated lamp elements in the presenceof hydrogen. In this experiment, the background gas flow was 1000 sccmof air at atmospheric pressure. The concentrations of hydrogen testedwere 1000 ppm, 4000 ppm, 1.6% and 3.2%, and each concentration wascycled on and off four times. The lamp element was powered at less thanfull rating, with 2 volts, and ˜0.3 amps. The wavelength of light shownin the graph of FIG. 6 is 963 nm. In general, the response was similarfor other wavelengths in the range measured.

[0124]FIG. 7 shows an expanded region of the data from FIG. 6 for thepurpose emphasizing the speed of response. At 3.2% hydrogen exposure,the response (increasing transmission) reaches 78% of its maximum in 12s, while the recovery (decreasing transmission) takes 85 s to reach 78%of its return to baseline. These times are significantly faster than theresponse and recovery times of unheated films, which are on the order of720 and 2400 s, respectively. It is also expected that these times couldbe further improved by empirical experimentation and optimization of thethin film processing by someone skilled in the art.

[0125]FIG. 6 also shows that concentrations of 1000 ppm were easilydetected, and that the response increases with increasing hydrogenconcentration. This increase is plotted in FIG. 8, which shows theresponse as a percent of the baseline as a function of hydrogenconcentration. This is particularly useful, as some sensor applicationsrequire output signals at different concentrations of hydrogen, e.g., asensing of 1% hydrogen might trigger a warning light, and a sensing of2% hydrogen might trigger a relay that shuts a system off. Extrapolationof the data suggests a lower detectable limit of 500 ppm. Alsoparticularly advantageous, is the magnitude of the response, which isgreater than 200% at 3.2% hydrogen concentration, and provides asubstantial signal to noise ratio. This is useful in minimizing falsealarms in a sensor device.

[0126] Although the invention has been variously disclosed herein withreference to illustrative embodiments and features, it will beappreciated that the embodiments and features described hereinabove arenot intended to limit the invention, and that other variations,modifications and other embodiments will suggest themselves to those ofordinary skill in the art. The invention therefore is to be broadlyconstrued, consistent with the claims hereafter set forth.

What is claimed is:
 1. A hydrogen gas detector for detection of hydrogengas in a gaseous environment, said detector comprising a light/heatsource, an optical detector, and an optical barrier therebetween,wherein the optical barrier responds to the presence of hydrogen byresponsively changing from a first optical state to a different secondoptical state, whereby transmission of light from said light/heat sourcethrough said optical barrier is altered by the presence of hydrogen andsaid altered transmission is sensed by said optical detector to providean indication of the presence of hydrogen gas in the gaseousenvironment.
 2. The hydrogen gas detector of claim 1, wherein the firstoptical state comprises a state of optical opacity of the opticalbarrier.
 3. The hydrogen gas detector of claim 1, wherein the secondoptical state comprises a state of optical non-opacity of the opticalbarrier.
 4. The hydrogen gas detector of claim 1, wherein the secondoptical state comprises translucency/transparency of the opticalbarrier.
 5. The hydrogen gas detector of claim 1, wherein the opticalbarrier comprises a rare earth metal thin film.
 6. The hydrogen gasdetector of claim 1, wherein the optical barrier comprises an yttriumthin film.
 7. The hydrogen gas detector of claim 1, wherein the opticalbarrier comprises a rare earth metal thin film deposited on the opticaloutput surface of the light/heat source, overlaid by a protective filmthat is permeable to hydrogen gas.
 8. The hydrogen gas detector of claim7, wherein the protective film comprises a palladium film.
 9. Thehydrogen gas detector of claim 7, wherein surface morphology roughnessof the optical output surface of the light/heat source prior todeposition of the rare earth metal thin film has been increased bytreatment of the optical output surface comprising a roughening stepselected from the group consisting of mechanical roughening, chemicalroughening, deposition of highly exfoliated or porous inorganicunderlayers, and deposition of porous polymer underlayers, to therebyincrease the response time of the rare earth metal thin film as comparedwith a corresponding unroughened optical output surface.
 10. Thehydrogen gas detector of claim 1, embodied as a unitary portablearticle.
 11. The hydrogen gas detector of claim 1, further comprising apower supply.
 12. The hydrogen gas detector of claim 11, wherein thepower supply comprises a battery.
 13. The hydrogen gas detector of claim1, further comprising an output module operatively coupled to theoptical detector, and arranged to provide an output alarm indicative ofthe presence or concentration of hydrogen gas in the gaseousenvironment.
 14. The hydrogen gas detector of claim 13, wherein saidoutput alarm is selected from the group consisting of visual, audibleand tactile alarms.
 15. The hydrogen gas detector of claim 1, whereinthe light/heat source comprises a lamp element providing heat outputincident to the generation of light.
 16. The hydrogen gas detector ofclaim 1, wherein the light/heat source comprises an incandescent lamp.17. The hydrogen gas detector of claim 16, wherein the incandescent lampcomprises a light-transmissive bulb, the exterior surface of which iscoated with a rare earth metal thin film as said optical barrier. 18.The hydrogen gas detector of claim 1, wherein the light/heat sourcecomprises an optical waveguide.
 19. The hydrogen gas detector of claim18, wherein optical output surface of said optical waveguide is coatedwith a rare earth metal thin film as said optical barrier.
 20. Thehydrogen gas detector of claim 18, wherein said optical barriercomprises a multi-layer structure deposited on the optical outputsurface of said optical waveguide, said multi-layer structure comprisingat least a first and a second layer, wherein the first layer absorbsoptical energy of a first wavelength and is thereby heated whileremaining transparent or translucent to optical energy of a secondwavelength, and the second layer comprises a rare earth metal thin film,the optical properties of which are responsive to the presence andconcentration of hydrogen gas in the surrounding environment.
 21. Ahydrogen gas detector for detection of hydrogen gas in a gaseousenvironment, said detector comprising a light/heat source, at least oneoptical detector, and at least one optical barrier deposed between thelight/heat source and each detector, wherein the optical barriersrespond to the presence of hydrogen by responsively changing from afirst optical state to a different second optical state, wherebytransmission of light from said light/heat source through said opticalbarriers is altered by the presence of hydrogen and said alteredtransmission is sensed by said optical detectors to provide anindication of the presence and concentration of hydrogen gas in thegaseous environment.
 22. The hydrogen gas detector of claim 21, whereinthe light/heat source comprises an incandescent bulb, and the opticalbarriers comprise a single rare earth metal thin film coating theexterior surface of said bulb, with a plurality of protective layersections overlaid on mutually exclusive portions of the rare earth metalthin film coating, wherein each protective layer section exhibits aunique permeability to hydrogen.
 23. The hydrogen gas detector of claim22, wherein the number of optical detectors exceeds the number ofheat/light sources, and wherein each optical detector is arranged inlight-receiving relationship with a heat/light source such that theluminous flux impinging each detector passes through only one of theplurality of protective layer sections.
 24. The hydrogen gas detector ofclaim 23, further comprising a plurality of output modules, each ofwhich is operatively coupled to an optical detector, and arranged toprovide an output indicative of the concentration of hydrogen gas in thegaseous environment.
 25. The hydrogen gas detector of claim 1, of ahand-held size and portable character.
 26. The hydrogen gas detector ofclaim 1, wherein a plurality of heat/light sources, optical detectors,and optical barriers, each set of which is arranged in operativerelationship to effect the detection of hydrogen gas in a gaseousenvironment, are further arranged in an array, and wherein eachcombination of heat/light source, optical detector, and optical barrieris configured to respond to a different concentration of hydrogen gas inthe gaseous environment.
 27. The hydrogen gas detector array of claim26, further comprising a corresponding array of output modulesoperatively coupled to the optical detectors, and arranged to provide anoutput indicative of the concentration of hydrogen gas in the gaseousenvironment.
 28. The hydrogen gas detector of claim 1, operativelycoupled with corresponding hydrogen gas detectors to form an extendedarea monitoring system for detection of hydrogen gas in the gaseousenvironment of said extended area.
 29. The hydrogen gas detector ofclaim 1, wherein the light/heat source comprises a lamp elementproviding heat output incident to the generation of light, with anyttrium thin film coated on said lamp element as said optical barrier,and a palladium film coated on the yttrium thin film.
 30. A method offabricating a hydrogen gas detector, comprising: providing a source ofluminous and thermal energy including an output surface for emittinglight and thermal energy; depositing on the output surface an opticalfilter comprising a rare earth metal thin film that responds to contactwith hydrogen by exhibiting a detectable change of opticaltransmissivity; positioning a light detector in light-sensing proximityto the source of luminous and thermal energy, whereby a change inoptical transmissivity of the rare earth metal thin film in exposure tohydrogen gas is detected as a change in luminous energy flux impingingon the detector, and outputting a signal indicative of said change inluminous energy flux.
 31. The method according to claim 30, wherein therare earth metal thin film is formed on the output surface of the sourceof luminous and thermal energy, by a technique selected from the groupconsisting of physical vapor deposition, chemical vapor deposition,sputtering, solution deposition, focused ion beam deposition, pulsedlaser deposition, electrolytic plating, and electroless plating.
 32. Themethod according to claim 30, wherein the rare earth metal thin film isformed on the substrate by physical vapor deposition.
 33. The methodaccording to claim 30, wherein the rare earth metal thin film is formedon the substrate by chemical vapor deposition using an organometallicprecursor that thermally decomposes to the metal hydride or elementalmetal in a reducing environment of hydrogen.
 34. The method according toclaim 30, wherein the outputting step comprises generating an outputselected from the group consisting of visual outputs, optical outputs,tactile outputs, electrical outputs and auditory outputs.
 35. The methodaccording to claim 30, wherein the rare earth metal thin film comprisesat least one metal selected from the group consisting of: (I) scandium,yttrium, lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, lutetium, actinium, thorium, protactinium, uranium,neptunium, plutonium, americium, curium, berkelium, californium,einsteinium, fermium, mendelevium, nobelium, and lawrencium, (II) alloysthereof, and (III) alloys containing one or more of such metals alloyedwith an alloying component selected from the group consisting ofmagnesium, calcium, barium, strontium, cobalt and iridium.
 36. Themethod according to claim 30, wherein the rare earth metal thin filmcomprises a rare earth metal component selected from the groupconsisting of trivalent rare earth metals reactive with hydrogen to formboth metal dihydride and metal trihydride reaction products, and whereinthe metal dihydride and metal trihydride reaction products havediffering optical transmissivity, and wherein transitions between themetal dihydride and metal trihydride reaction products are caused by thepresence or absence of hydrogen gas contacting the rare earth metal thinfilm.
 37. The method according to claim 30, wherein the rare earth metalthin film is overlaid by a hydrogen-permeable material comprising ametal selected from the group consisting of Pd, Pt, Ir, Ag, Au, Ni, Co,and alloys thereof.
 38. The method according to claim 30, wherein therare earth metal thin film is overlaid by a hydrogen-permeable materialthat is doped with a dopant selected from the group consisting of Mg,Ca, Al, Ir, Ni and Co.
 39. The method according to claim 30, wherein therare earth metal thin film is overlaid by a thin film of a materialincluding a metal selected from the group consisting of palladium,platinum, and iridium.
 40. The method according to claim 30, wherein therare earth metal thin film comprises yttrium.
 41. The method accordingto claim 30, wherein the rare earth metal thin film comprises a metalselected from the group consisting of lanthanum and yttrium, and therare earth metal thin film is formed on the output surface of the sourceof luminous and thermal energy by CVD utilizing a correspondingprecursor, wherein said precursor is selected from the group consistingof tris(cyclopentadienyl)lanthanum, tris(cyclopentadienyl)yttrium,β-diiminate complexes of lanthanum, β-diiminate complexes of yttrium,lanthanum amides, and yttrium amides.
 42. The method according to claim30, wherein the rare earth metal thin film comprises lanthanum, and therare earth metal thin film is formed on the substrate by CVD utilizing aprecursor, wherein said precursor is selected from the group consistingof La(NR₂)₃, La(NR₂)₃.L, La(R)₃ and La(R)₃.L, wherein R is selected fromthe group consisting of C₁ to C₈ alkyl and C₁ to C₈ aryl and L is aLewis base ligand selected from the group consisting of amines, arylsand aryl amines.
 43. The method according to claim 42, wherein the Lewisbase ligand is selected from the group consisting of NH3, primaryamines, secondary amines, tertiary amines, polyamines.
 44. The methodaccording to claim 43, wherein the Lewis base ligand is selected fromthe group consisting of pyridine, methylamine, dimethylaminetrimethylamine, dimethylethylamine, N,N,N′,N′-tetramethylethylenediamineand N,N,N′,N′N″-pentamethyldiethylenetriamine.
 45. The method accordingto claim 30, wherein the rare earth metal thin film comprises yttrium,and the rare earth metal thin film is formed on the substrate by CVDutilizing a precursor, wherein said precursor is Y(NSiR′₃)₃, wherein R′is selected from the group consisting of C₁ to C₈ alkyl and C₁ to C₈aryl.
 46. The method according to claim 30, wherein the rare earth metalthin film has deposited thereon an overlayer comprising palladium, andsaid overlayer is formed by chemical vapor deposition on the rare earthmetal thin film using a palladium precursor selected from the groupconsisting of Pd(hfac)₂, Pd(allyl)₂, CpPd(allyl), Pd(allyl)(hfac),COD.Pd(Me)₂ and Pd(methylallyl)(hfac).